Btrfs: fix possible memory leak in scrub_setup_recheck_block()

bbio has been malloced in btrfs_map_block() and should be
freed before leaving from the error handling cases.

spatch with a semantic match is used to found this problem.
(http://coccinelle.lip6.fr/)
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>

fs/btrfs/scrub.c

@@ -1029,6 +1029,7 @@ static int scrub_setup_recheck_block(struct scrub_dev *sdev,
 				spin_lock(&sdev->stat_lock);
 				sdev->stat.malloc_errors++;
 				spin_unlock(&sdev->stat_lock);
+				kfree(bbio);
 				return -ENOMEM;
 			}
 			sblock->page_count++;